This article was published as part of the

2009 Themed issue dedicated to Professor Jean-Pierre Sauvage

Guest editor Professor Philip Gale

Please take a look at the issue 6 table of contents to access

the other reviews.

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Fullerene for organic electronicsw

Dirk M. Guldi,*aBeatriz M. Illescas,

bCarmen M

aAtienza,

bMateusz Wielopolski

a

and Nazario Martın*bc

Received 20th January 2009

First published as an Advance Article on the web 1st May 2009

DOI: 10.1039/b900402p

This tutorial review surveys and highlights the integration of different molecular wires—in

combination with chromophores that exhibit (i) significant absorption cross section throughout

the visible part of the solar spectrum and (ii) good electron donating power—into novel electron

donor–acceptor conjugates. The focus is predominantly on charge transfer and charge transport

features of the most promising systems.

1. Introduction

Donor–bridge–acceptor (DBA) architectures have emerged as

suitable models for probing electron transfer processes at the

molecular level. In such, the bridge is assumed to mediate

charge transfer between the donor and the acceptor. One of

the most striking scenarios implies wire-like behavior. In other

words, electron transfer processes (i.e., charge separation and

charge recombination) depend either weakly on distance or

completely lack any dependence. In donor–acceptor conju-

gates, visible light excitation, for instance, may result in a

charge transfer—mediated by the bridge—from the photo-

excited donor to the acceptor or from the donor to the

photoexcited acceptor. The kinetics of both processes, namely,

charge separation and/or charge recombination are then

reflected by the electron transfer rate constant:

kET = k0exp(�bRDA)

where k0 is a kinetic prefactor/preexponential factor, RDA

represents the donor–acceptor distance and b is the so-called

attenuation factor or dumping factor. At first glance, bquantifies the charge transfer capability of the bridge and

therefore becomes a bridge specific parameter, which depends

on the magnitude of the coupling between the donor and

acceptor sites and the energy of the electron (or hole) transfer

states localized on each site. Equally important, the connec-

tivity patterns between donor, bridge and acceptor highly

impact the electron transfer rates. Overall, b has been imple-

mented to assess wire-like behavior in a wide variety of

aDepartment of Chemistry and Pharmacy, Interdisciplinary Center forMolecular Materials (ICMM) Fiedrich-Alexander-UniversitatErlangen-Nurnberg, Egerlandstr. 3, D-91058 Erlangen, Germany.E-mail: dirk.guldi@chemie.uni-erlangen.de;Fax: +49-913-185-28307

bDepartamento de Quımica Organica, Facultad de Quımica,Universidad Complutense, E-28040, Madrid, Spain.E-mail: nazmar@quim.ucm.es; Fax: +34-91-394-4103

c Instituto Madrileno de Estudios Avanzados en Nanociencia(IMDEA-Nanociencia), Campus UAM, Cantoblanco, E-28049,Madrid, Spainw Dedicated to Professor Jean-Pierre Sauvage on the occasion of his65th birthday.

Dirk M. Guldi

Dirk M. Guldi graduated fromthe University of Cologne(Germany) in 1988, fromwhere he received his PhD in1990. In 1992, after a post-doctoral appointment atthe National Institute of Stan-dards and Technology, he tooka research position at theHahn-Meitner-Institute, Berlin.In 1995 he joined the faculty ofthe Notre Dame RadiationLaboratory where he was pro-moted to Associate Scientist in1996. In 1999 he completed hisHabilitation at the University

of Leipzig (Germany). Since 2004 he has been appointed asProfessor of Physical Chemistry at the Friedrich-AlexanderUniversity in Erlangen (Germany). He was awarded with theJSPS Fellowship (2003; The Japan Society for the Promotionof Science) and JPP-Award (2004; Society of Porphyrins andPhthalocyanines).

Beatriz M. Illescas

Beatriz M. Illescas obtainedher degree in Chemistry atthe Universidad Complutenseof Madrid, in 1992, where shereceived her PhD degree in1998 under the direction ofProfessors C. Seoane andN. Martın. In 1998 sheworked as postdoctoralresearcher on the synthesis offullerenes with pharma-cological activity, in a projectwith Janssen-Cilag S.A. Since1999 she has been appointed asan assistant professor at theUniversidad Complutense of

Madrid. Her current interests focus on the covalent and supra-molecular chemistry of fullerenes and TTFs.

This journal is �c The Royal Society of Chemistry 2009 Chem. Soc. Rev., 2009, 38, 1587–1597 | 1587

TUTORIAL REVIEW www.rsc.org/csr | Chemical Society Reviews

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donor–acceptor conjugates. Typical values for b range, on one

hand, from 1.0 to 1.4 A�1 for protein structures and, on the

other hand, from 0.01 to 0.04 A�1 for highly p-conjugatedbridge structures.1 In vacuum, values of b are relatively large

in the range of 2.0 to 5.0 A�1. Moreover, several other

parameters exert an impact on the electron transfer rate. These

are in particular the underlying driving forces (�DG1), the

corresponding reorganization energies (l), and the electronic

couplings that exist between electron donor and electron

acceptor (V).

With the aforementioned in hand, we may address a series of

basic requirements for the design of molecular systems that

exhibit efficient wire-like behavior: (i) matching of energy levels

(orbitals) between donor/acceptor and the bridge sites, (ii) strong

electronic couplings between the electron donor and acceptor

units via the bridge orbitals, and (iii) small attenuation factors.

As a matter of fact, recent strategies have built on the

incorporation of components that exhibit wire-like behavior

into electron donor/acceptor structures to improve the overall

performance features—ranging from charge separation/charge

recombination dynamics to charge separation quantum yields.

These molecules play an overriding role in a number of

important applications—electrical conductors,2,3 photovoltaic

cells,4,5 electroluminescent devices,6,7 nonlinear optics,8 and

field-effect transistors.9 Therein, long-range electron/energy

transfer processes constitute a common mode of operation.

The thrust of this review is to survey, highlight and compare

methods that are currently employed to probe molecular

systems or fragments with respect to their capability to

efficiently transport charges along the molecular framework

from one site to another.

To this end, two different strategies have been implemented.

The first of these is based on the covalent linkage of electron

donors to electron acceptors by molecular wires. A certain

degree of charge delocalization, that is, a transfer of charge

density from the donor to the acceptor, may occur in the

ground state. Nevertheless, we will focus on electron donor/

acceptor structures, where irradiation with light induces the

promotion of electrons from the donor to the acceptor to yield

a spatially separated radical ion pair. For non-fullerene-

containing conjugates, this topic has been extensively reviewed

in literature.10 Thus, we will limit ourselves to the description

of examples where fullerenes play an integrative role. The

second strategy involves embedded fullerenes and fullerene

derivatives between two electrodes. Applying an external

potential to the electrodes leads to conductance along the

molecular framework. It is important to note, however, that

the presence of an electron donor unit linked to a fullerene

moiety by a conjugated spacer does not necessarily guarantee

the formation of a charge separated state. Instead, energy

transduction or even competitive simultaneous or sequential

energy and electron transfer processes may occur.

Carmen Ma Atienza obtained

her BSc (1999) and DEA

(2001) from the University of

Autonoma of Madrid. Then

she moved to the University

Complutense of Madrid

(UCM) where she obtained

her PhD in 2005 under the

supervision of Profs N. Martın

and C. Seoane. In 2005 she

worked as a postdoctoral

researcher with Prof. D. M.

Guldi in Erlangen-Nurnberg

University on photophysics

study of electron transfer in electroactive organic systems. After

a second postdoctoral stay at University of Castilla La Mancha,

she returned to UCM, where she currently enjoys a visiting

professor contract.

Carmen MaAtienza

Mateusz Wielopolski

Mateusz Wielopolski was bornin Katowice in Poland in 1981.He moved to Germany in theearly 1990s, where he com-pleted his education. In 2006,he received his Master ofScience in Molecular NanoScience from the Universityof Erlangen (Master ThesisAward). He obtained hisPhD two and a half years laterin February 2009 at the Uni-versity of Erlangen for hisresearch on molecular wires.

Nazario Martın

Prof. Nazario Martın is afull professor at UniversityComplutense of Madrid(UCM). He worked as a post-doctoral fellow (1987–1988)at the Universitat Tubingenwith Michael Hanack onelectrically conducting organicmaterials. In 1994, he was avisiting Professor at the Uni-versity of California, SantaBarbara (UCSB) workingwith Fred Wudl on fullerenes,and in 2005 at the Universitiesof California, Los Angeles(UCLA) and Angers

(France). Professor Martın’s research interests include thechemistry of carbon nanostructures, p-conjugated systems, andelectroactive molecules in the context of electron transfer pro-cesses, photovoltaic applications and nanoscience. He is a Fellowof the Royal Society of Chemistry and the President of theSpanish Royal Society of Chemistry.

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An intriguing example are phenyleneethynylene oligomers of

varying length (1a–c) that connect a fullerene to a ruthenium

trisbipyridine.11 These oligomers show transduction of energy—

starting with the excited ruthenium MLCT state, forming inter-

mediately the p–p* state of the oligomer and relaying it to the

fullerene—that is nearly distance independent (Fig. 1), even for

edge-to-edge distances up to 2.3 nm. Overall, the experiments

suggest an interesting alternative for long-range energy transfer

in which excited-state energy is efficiently transferred via near-

isoenergetic bridge states. Thereby, the intermediately populated

bridge states, which decrease in energy with increasing spacer,

may mediate constant energy transfer rates.

To contrast the interplay between competitive energy and charge

transfer processes, several phenylenevinylene-based dendrons

bearing dibutylaniline (2a,b) or dodecyloxynaphthalene (3a,b)

electron donors have been designed.12,13 Efficient and rapid

energy transfer dominates the intramolecular deactivation of

the excited dendrons, generating the fullerene singlet excited

state in nearly quantitative yields. A spectroscopic and kinetic

analysis, however, confirms the presence of an alternative

intramolecular charge transfer reaction which yields the

C60��–dendron�+ radical ion pair state. It is the energy

gap—the energetic positioning of the radical ion pair state

relative to the singlet excited states of the dendron and fullerene—

that controls the outcome, namely, either a sequential (2a,b) or

a competitive (3a,b) scenario for the energy and charge

transfer processes.

In summary, this review is aiming to demonstrate that

utilizing the unique electronic features of fullerenes help to

determine and to compare the b values of a variety of structu-

rally different wire-like architectures, which are of particular

interest for molecular electronics. Furthermore, it paves the way

for the evaluation of molecular components (i.e. fullerene

derivatives, molecular wires, etc.) with respect to their perfor-

mance in molecular devices and molecular electronic circuits.

2. C60–wire–donor systems

The molecular bridge which links the donor to the acceptor is

considered to play a vital role in the light of several key

aspects.14–16 First, covalent linkers eliminate diffusion as the

rate determining step of the charge transfer process. This helps

to accelerate the electron transfer dynamics. Second, the

chemical nature and the length of the bridge presides over

the donor–acceptor separation, orientation, overlap and

topology. Implicit here, a structurally rigid bridge prevents

unrestrained and undesired rearrangements. Structural

flexibility, on the other hand, may lead to different photo-

reactivities (i.e., reaction rates, pathways, products, etc.).

Third, the chemical nature of the bridge (e.g. p-conjugation)strongly affects the conductance/charge-transport behavior.

Finally, the influence of the bridge on the electronic nature

of the donor/acceptor pair is supposed to be negligible, and the

coupling of the bridge to the donor/acceptor sites is propor-

tional to the overlap between their electronic clouds.

2.1 C60–wire–ZnP systems

Fig. 2 illustrates a few leading examples, in which a ZnP donor

and C60 are connected through diverse bridges. These com-

prise either (i) a –CO–NH– amide bond (4) (in THF: kCS =

2.2 � 1010 s�1; kCR = 2.0 � 106 s�1), (ii) a –NH–CO– reversed

amide bond (5) (in THF: kCS = 1.7 � 1010 s�1; kCR = 3.7 �105 s�1), (iii) a –CRC– triple bond (6) (in THF: kCS = 3.7 �1010 s�1; kCR = 1.5 � 106 s�1) or (iv) a –NQN– double bond

(7) (in THF: kCS = 7.2 � 109 s�1; kCR = 6.8 � 106 s�1).17,18

Although the large donor–acceptor distances are comparable

in (i) to (iv), different electronic couplings were observed, as

indicated by varying charge separation and charge recombina-

tion features (i.e., rates, efficiencies, quantum yields, etc.). The

p-stacked ZnP–C60 conjugate may be considered as a point of

reference. Here, van der Waals contacts promote significantly

faster picosecond dynamics.19

In the aforementioned DBA conjugates the efficiency of the

charge transfer process decreases exponentially with increasing

bridge length. Long-range electron transfer processes, which in

these conjugates occur via superexchange, are typically limited

to distances of approximately 20 A. Above these distances the

rate-determining electronic coupling is largely diminished and,

consequently, insufficient to compete with the intrinsic excited

state deactivation of the electron donor.

Fig. 1 Examples of D–oligomer–C60 (1a–c) and D–dendrimer–C60 (2a,b and 3a,b) systems.

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The transport of charges over distances beyond 20 A

requires, however, alternative concepts. This task becomes

particularly relevant for the generation of ultralong radical

ion pair lifetimes. One viable option to achieve charge separa-

tion over large distances utilizes a relay of several short-range

electron-transfer events along well-designed redox gradients

rather than transferring the electrons in one single, concerted

long-range step.

Such a ‘‘relay concept’’ was successfully realized by combin-

ing several different redox-active building blocks–ferrocene (Fc)

to ZnP to free-base tetraphenylporphyrin (H2P) and to C60—to

form Fc–ZnP–H2P–C60 and Fc–ZnP–ZnP–C60 conjugates.20,21

Fig. 3 summarizes some leading examples, where the first

electron donor units (i.e., Fc) and the ultimate electron accep-

tors (i.e., C60) are separated by nearly 50 A. In these novel

molecular architectures we demonstrated very slow intramole-

cular charge recombination processes—observable only by ESR

measurements in frozen matrices under light irradiation. When

comparing Fc–ZnP–H2P–C60 (0.38 s) or Fc–ZnP–ZnP–C60

(1.6 s) conjugates with analogous H2P–C60 or ZnP–C60

conjugates, the difference in radical ion pair lifetimes amounts

to six orders of magnitude, i.e. seconds vs. microseconds.

Primarily, low electronic coupling elements guarantee

such outstanding lifetimes. The couplings, V, are as small as

(5.6 � 0.5) � 10�5 cm�1 relative to 7.9 � 1.7 cm�1 seen for the

ZnP–C60 conjugates, where the electron–donor–acceptor

separations are B12 A. These findings correlate well with

negligible orbital overlap—an argument that finds further

support when calculating the attenuation factor (b). A plot

of the radical ion pair lifetimes of Fc–ZnP–H2P–C60 (B50 A),

ZnP–H2P–C60 (B30 A) and H2P–C60 (~12 A) vs. electron–

donor–acceptor separation is well fitted with a straight line

and yields a b value of 0.60 A�1.

Importantly, such a b value is located within the boundaries

of nonadiabatic electron transfer reactions for saturated

hydrocarbon bridges (0.8–1.0 A�1) and unsaturated phenylene

bridges (0.4 A�1).22,23

In light of long-range electron-transfer processes, para-

conjugated molecular wires emerged as particularly striking

Fig. 3 Structures of Fc–ZnP–H2P–C60, and Fc–ZnP–ZnP–C60 conjugates.

Fig. 2 Chemical structures of ZnP–C60 conjugates 4–7.

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candidates, since they seem not to actively participate in

the electron-transfer events (i.e., charge separation, charge

recombination, charge hopping, etc.). In other words, para-

conjugated molecular wires play a real mediating role in the

transport of charges due to their well-isolated high-lying

LUMO and/or low-lying HOMO orbitals.

p-Phenylenevinylene, alkyne and p-phenyleneethynylene

oligomers turned out to be versatile model bridges with

chemically tailored properties. Accordingly, we and others

have tested a series of novel electron-donor–acceptor conju-

gates that incorporate ZnP as electron donors and C60 as

electron acceptors, linked by p-phenylenevinylene (ZnP–

p-phenylenevinylene–C60),24 alkyne (ZnP–alkyne–C60),

25

and p-phenylenebutadiynylene oligomers (ZnP–p-phenylene-

butadiynylenes–C60) of variable length26 (see Fig. 4). Notable

in this context is the connection of p-phenylenevinylenes,

alkynes and p-phenylenebutadiynylenes to the ZnP’s phenyl

group. The following aspects were considered at the forefront

of our investigations: (i) systematic variation of the length of

the p-conjugated system; (ii) determination and evaluation of

the structural (i.e., donor–acceptor separation) and electronic

(i.e., energy levels) effects on the charge-transfer rates;

(iii) testing the molecular-wire behavior in p-phenylenevinylene,

alkyne or p-phenylenebutadiynylene based DBA ensembles in

terms of attenuation factors.

For ZnP–p-phenylenevinylene–C60 (11a,b), a detailed

physico-chemical investigation—probing mainly long-range

charge separation (in THF: rate = 3.9 � 0.6 � 109 s�1)

and charge recombination events (in THF: rate = 1.05 �0.15 � 106 s�1) including their kinetics—revealed attenuation

factors of 0.03 � 0.005 A�1. Even in comparison to

p-phenylene (i.e., 0.32–0.66 A�1), polyene (i.e., 0.04–0.2 A�1)

and polyyne (i.e., 0.04–0.17 A�1), such b values are extra-

ordinary small.27–30 Vital for the wire-like behavior is that the

energies of the HOMO of C60 match those of the long

p-phenylenevinylene bridges. This facilitates charge injection

into the wire. Equally important is the strong electronic

coupling, realized through the para-conjugation in p-phenyl-

enevinylenes. This leads to donor/acceptor coupling constants

of B2.0 cm�1—even at electron donor–acceptor separations

of 40 A—and assists electron transfer reactions that reveal

shallow distance dependences. Remarkable is the fact that

these features are realized despite the rotational freedom of the

donor–bridge and bridge–acceptor contacts. To analyze the

charge recombination mechanism the radical pair lifetimes

were probed between 268 and 365 K. The Arrhenius plots

can be separated into two distinct sections: the low-temperature

regime (i.e., o300 K) and the high-temperature regime

(i.e., 4300 K). The weak temperature dependence in the 268

to 320 K range suggests that a stepwise charge recombination

may be ruled out, leaving electron tunneling via superexchange

as the operative mode. This picture is in sound agreement with

the thermodynamic barrier, necessary to overcome in forming

ZnP–p-phenylenevinylene�+–C60��. At higher temperatures

(i.e., 4300 K) the situation changes and the charge recombi-

nation is accelerated. The observed strong temperature

dependence suggests a thermally activated charge recombina-

tion. The activation barriers (Ea), derived from the slopes

(i.e., between 0.2 eV), confirm the HOMO (C60) � HOMO

(wire) energy gap.

Somewhat larger are, however, the attenuation factors

(i.e., 0.06 � 0.005 A�1) determined for polyalkyne bridges in

ZnP–alkyne–C60 (12a,c) with charge separation and charge

recombination rates in the order of 7.5 � 2.4 � 109 s�1 and

1.6 � 0.2 � 106 s�1, respectively. Nevertheless, these findings

prove that even triple bonds are effective mediators of long-

range electronic interactions up to nearly—but not limited

to—24 A. An interesting aspect of this work was that the

direct linkage between the polyalkyne wires and C60 provides

much better bridge-acceptor contacts.

Electron transfer features have also been successfully

demonstrated for a ZnP–p-phenylenebutadiynylene–C60 series

(13a,c) with effective center-to-center donor–acceptor dis-

tances ranging from 20 to 40 A. In comparison with the

ZnP–p-phenylenevinylene–C60 and ZnP–alkyne–C60 conju-

gates, the significantly higher attenuation factor of 0.25 A�1

prompts to a less effective superexchange mechanism due to

the alternating bond lengths—double vs. triple bonds.

The aforementioned results corroborate effective charge

mediating properties of p-phenylenevinylene, alkyne and

Fig. 4 Chemical structure of some ZnP–p-phenylenevinylene–C60, ZnP–alkyne–C60 and ZnP–p-phenylenebutadiynylene–C60 electron donor–

acceptor conjugates.

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p-phenylenebutadiynylene oligomers. Nonetheless, some

subtle differences emerge. To shed light onto this aspect, we

have varied the substitution pattern of ZnP.31 In particular,

placing the polyalkyne at the b-position (14a,c) notably affects

the electronic coupling. Examining the molecular orbitals

suggests a strong propensity for intramolecular electron trans-

fer from ZnP to C60 following photoexcitation. To this end,

the b-substituted polyalkyne gives rise to a much faster charge

recombination (in THF: 1.0 � 0.5 � 1010 s�1) process due, at

least in part, to more favorable orbital interactions between

ZnP and C60. However, no attenuation factor was determined.

Quite a contrasting picture was found in b-substitutedZnP–p-phenyleneethynylene–C60 (15a,b) (Fig. 5). Specifically,

charge separation occurs in polar media affording a charge

separated state but the lifetimes tend to increase with increas-

ing distance between the two photoactive units. In regard to

donor–acceptor separations of up to 23 A, it is safe to assume

a through-bond mechanism, where the bridge plays a crucial

role. The studied conjugates indicate a linear dependence of

the electron transfer rate constant on the donor–acceptor

distance and yield an attenuation factor of 0.11 A�1. Again,

implementing triple bonds between the phenyl groups impacts

the extended p-conjugation and, in turn, the charge transfer

properties.

Furthermore, solely one analogous ZnP(H2P)-p-phenylene-

vinylene–C60 (16a,b)—2-mer—has been synthesized and

investigated.32 Photophysical studies revealed that efficient

electron transfer processes upon photoexcitation afford the

corresponding radical ion pairs of ZnP(H2P)–p-phenylene-

vinylene–C60. The results confirm that b-substitution on the

porphyrin moiety favors the electronic coupling and the

electronic communication between the donor and acceptor

units. Work is currently in progress to prepare related

conjugates with varying length of the p-phenylenevinylene

bridge to determine the attenuation factor in these b-substitutedconjugates.

We encounter a quite different situation in H2P–thiophene–

C60 conjugates (17a,c) (Fig. 6). Here, different thiophene

oligomers are directly linked to the meso position of H2P

affording electron donor–acceptor distances of up to

55.7 A.33,34 Furthermore, the electron donating properties of

polythiophene alter the photoreactivity in H2P–thiophene–

C60, i.e., the bridges actively participate in the electron transfer

processes. Not surprisingly, the attenuation factors varied with

solvent polarity: 0.11 A�1 in o-dichlorobenzene and 0.03 A�1

in benzonitrile.

Common to the aforementioned conjugates is that in

temperature dependent measurements the charge recombina-

tion kinetics imply an efficient decoupling of the donor–bridge

and bridge–acceptor contacts, which leads to a significant

slow-down of the electron transfer rates. Reversible interrup-

tion of the p-conjugation through temperature-induced rota-

tions along the wire axis is believed to be responsible for this

effect.

Oligothienylenevinylenes (nTV) of different length have also

been reported as molecular wires which connect ZnP to C60.35

For 18a,b (Fig. 6) with n = 1, energy transfer prevails over

charge separation in nonpolar solvents. In polar PhCN, on the

other hand, charge-separation occurs predominantly due to

the stabilization of the radical ion pair state by polar solvent

molecules. In case of longer nTV bridges (n= 3) and nonpolar

solvents, charge separation surpasses energy transfer. The

decrease of kCS with increasing length of the wire gives rise

to a relatively small damping factor for the charge-separation

process.

Enthrallingly challenging is the incorporation of oligomeric

bridges, in which the p-conjugation is irreversibly broken off

by, for instance, the chemical nature of the bridge structure.36

Chiral binaphthyl derivatives meet such criteria and are also

used as electroactive species (Fig. 7). In contrast to conjugated

p-phenylenevinylene oligomers, the p-conjugation between the

two naphthyl units is effectively disrupted via atropisomerism.

Consequently, distances and electronic interactions between

donor and C60 are drastically changed.

In fact, we have found that topological effects of the

geometrically well-defined chiral binaphthyl spacer play a

leading role in the electronic interactions in these donor–

acceptor ensembles. Thus, in ZnP–binaphthyl–C60 (19),Fig. 5 b-Substituted ZnP–p-phenyleneethynylene–C60 and ZnP(H2P)–

p-phenylenevinylene–C60.

Fig. 6 H2P–oligothiophene–C60 and ZnP–oligothienylenevinylene–C60 donor–acceptor conjugates.

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associative p–p interactions augmented by electron transfer

interactions favor a conformer in which the ZnP is close to

C60. Such a conformation gives rise to appreciable through-

space electronic communication with charge recombination

dynamics in the order of B106 s�1.

Reversible interruption of p-conjugation, on the other hand,

may be used to control the electron transfer properties of mole-

cules (gate function). In this context, Ito et al.37 reported the

synthesis of a H2P–sexithiophene–C60 (20) where the two central

thiophene units of the sexithiophene spacer are bridged by a

crown-ether-like polyether chain. In 20, the photoinduced electron

transfer is entirely governed by complexation/decomplexation of a

sodium cation in the crown ether ring. As a consequence it

operates as a complexation-gated molecular wire (Scheme 1).

In conclusion, the investigations focusing around ZnP–C60

electron donor–acceptor ensembles were ground breaking by

revealing attenuations factors that varied with the nature of

the oligomers. A big plus of ZnP–C60 conjugates is their

simplicity in photoactivity, namely, selective ZnP photoexita-

tion is followed by intramolecular charge separation and

charge recombination. Nevertheless, structural aspects such

as the ZnP substitution pattern (i.e., meso-substitution or

b-substitution) seem to highly impact the underlying charge

transfer processes.

2.2 C60–wire–exTTF systems

Bearing the aforementioned conjugates in mind, C60–exTTF

conjugates offer the great advantage of linking the oligomeric

bridge to exTTF without compromising the p-conjugation(Fig. 8). Particularly, extended p-conjugation is ensured

between the anthracenoid part of the exTTF-donor,

the oligomeric bridge (i.e., p-phenylenevinylenes, p-phenylene-

ethynylenes or oligofluorenes), and the pyrrolidine ring of the

C60 derivative.

C60–p-phenylenvinylene–exTTF systems (21)38,39 exhibit

charge separation processes over distances of up to 50 A.

The outcome is the respective radical ion pair. Interestingly,

the radical ion pair lifetimes are in the range of 465 ns to

557 ns, which indicates shallow dependence of the electron-

transfer rate on the length of the oligomeric bridge. In the

corresponding 7-mer the radical ion pair lifetimes turned out

to be 10-times longer, which has been attributed to the loss of

planarity of the bridge moiety. The dihedral angles between

the two terminal benzenes were calculated to 381. Plotting the

electron transfer kinetics as a function of donor–acceptor

distance led to linear dependences in THF and benzonitrile.

An extraordinarily small attenuation factor of only

0.01 � 0.005 A�1 was determined from the slope of these

plots. Pivotal for the observed wire-like behavior is the

coupling constant (V) with values of B5.5 cm�1, which is

peculiarly strong considering that the donor and acceptor sites

are separated by a distance of 40 A.

Similarly, in C60–p-phenyleneethynylene–exTTF systems

(22)40,41 transient absorption spectroscopy confirmed the

presence of spectral signatures of the one-electron oxidized

exTTF�+ radical cation and the one-electron reduced C60��

radical anion. These findings hold, however, only for the 1-mer

and 2-mer, while in the 3-mer no radical ion pair formation

was evidenced.

Relating the charge separation and charge recombination

dynamics in THF to the donor–acceptor distance provided an

attenuation factor for the p-phenyleneethynylene bridges of

0.2 � 0.05 A�1. Extrapolating the linear relationship to the

center-to-center distance of the 3-mer results in a charge

separation rate that would be notably slower than the intrinsic

deactivation of the C60 singlet excited state. This, in turn, helps

to rationalize the lack of electron transfer in the 3-mer. To

shed light onto the higher values of the attenuation factor, we

conducted calculations that revealed a few trends. First, the

molecular geometry of the C60–p-phenyleneethynylene–exTTF

conjugates does not exhibit a significant deviation from

planarity (less than �121), thus allowing the electronic

coupling between the donor and acceptor units. Secondly,

scrutinizing the electronic structure unveils that the HOMO

in C60–p-phenylenevinylene–exTTF (21) reaches into the

p-phenylenevinylene bridge, whereas the HOMO in the

C60–p-phenyleneethynylene–exTTF (22) is completely loca-

lized on the exTTF. Thus, charge injection into the bridge is

much easier in 21 than in 22. Thirdly, local electron affinity

mappings evidence a homogenous distribution of electron

affinity throughout the whole bridge in C60–p-phenylene-

vinylene–exTTF, whereas in the p-phenyleneethynylene

systems local maxima were found on the phenyl rings and

minima on the triple bonds. This points to the polarizing

character of the triple bonds and their shorter bond length.

In comparison to the pure double bond character of the

p-phenylenevinylene bridges, the bond length alternation

caused by the presence of triple bonds in C60–p-phenylene-

ethynylene–exTTF (22) disrupts the extended p-conjugationand strongly influences the charge separation.

Fig. 7 Chemical structure of ZnP–binaphthyl–C60.

Scheme 1 Sexithiophene as a complexation-gated molecular wire in

H2P–sexithiophene–C60 systems.

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Finally, from the linear dependence of the rate constants

(i.e., 109 s�1 for charge separation and 105 s�1 for charge

recombination) on distance in C60–oligofluorene–exTTF

(23)42 (Fig. 8) an attenuation factor of 0.09 A�1 emerged. In

other words, the ability to conduct charges in oligofluorenes

lies between that of p-phenylenevinylenes and that of

p-phenylenethynylenes. DFT calculations predict fairly good

electronic communication between exTTF and C60. The

HOMO and LUMO energies of exTTF and the oligofluorene

building blocks, respectively, evidently match each other,

providing a good orbital overlap between the donor and the

bridge. Electron affinity calculations also confirm the electron

transfer pathway from the donor over the bridge to the

fullerene acceptor in C60–oligofluorene–exTTF. A homo-

genous distribution of electron density throughout the whole

molecule and a channel of high local electron affinity through

the bifluorene bridge that maximizes at C60 prove the charge-

transfer features of these systems.

2.3 C60–wire–Fc systems

An interesting alternative involves testing the charge-transfer

efficiencies of polyporphyrin bridges that are linked via

butadiynes (Fig. 9).43 In that sense a series of Fc–porphyrin–

C60 conjugates (24a,c) (i.e., 1-mer, 2-mer and 4-mer) were

investigated. Photoexcitation of the polyporphyrin results in

the formation of spatially separated radical ion pair states.

The formation occurs via a sequence of electron transfer steps.

Charge separation and charge recombination rates were

elucidated utilizing transient absorption and fluorescence

spectroscopic tools. The charge recombination rates are

remarkably fast (i.e., 15–1.3 � 108 s�1)—a consequence that

stems from bridge-mediated electronic couplings. Plotting the

charge recombination rates vs. donor–acceptor distances fails

in providing a straightforward relationship. Nevertheless,

it is plausible to separate the data points into two groups.

The slope of a line that connects the first two data points

(i.e., shorter bridges) corresponds to an attenuation factor of

0.18 A�1. This value is in the lower end of those typically

found for conjugated bridge structures and indicates weak

distance dependence. On the other hand, connecting the

second and third data points (i.e., longer bridges) results in a

line with practically no distance dependence (b = 0.003 A�1).

The long-range charge recombination process seems to occur

via electron tunneling rather than via hopping or triplet

recombination. Obviously, the distance dependence of the

electron transfer processes in these conjugates should not be

considered as a single parameter for the charge-transfer effi-

ciency and the exact mechanism still remains to be elucidated.

However, the observation that the 4-mer mediates long-range

charge transfer over a distance of 65 A is essential for the

application of such structures as molecular wires.

The use of p-conjugated oligomers to link fullerenes to

electron donating ferrocenes leads to further examples of

donor–acceptor conjugates. Hereby, the radical cations are

delocalized over the donor and the linker. This, in turn, affects

the charge separation and charge recombination dynamics in a

favorable manner. However, no distance dependence has been

tested in such systems.44,45

Extending the scope of earlier studies on 25 (Fc–nT–C60)

paved the way for recent studies on 26 (Fc–tm-nT–C60)

(Fig. 10).46 In the latter, a trimethylene (tm) fragment has

been inserted between the ferrocene and the oligothiophene

moieties, which perturbs the conjugation between the two

chromophores. Time-resolved fluorescence and transient

absorption spectroscopy of Fc–nT–C60 disclosed that the

excited state deactivation in non-polar toluene occurs exclu-

sively via energy transfer. This process evolves from the 1*nT

formation and results in the generation of 1*C60. In polar

benzonitrile, on the other hand, charge separation is thermo-

dynamically favored. A radical cation is generated instan-

taneously. Interestingly, the positive charge is delocalized over

the Fc and nT moieties ((Fc–nT)�+–C60��). Upon systematic

variation of the spacer length, nT, from 4T to 12T, the

lifetimes of (Fc–nT)�+–C60�� increased accordingly from 0.1

to 50 ns. In general, the varying oxidation potentials of the nT

Fig. 9 Fc–ZnP–C60 electron donor–acceptor systems.

Fig. 10 Chemical structures of Fc–nT–C60 (25) and Fc–tm-nT–C60

conjugates and (26).

Fig. 8 Examples of exTTF–oligomer–C60.

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moieties seem to control the electron transfer processes.

Considering the tm-extended conjugates, Fc–tm-nT–C60, a

change in reactivity was observed in polar solvents. Charge

separation—to yield initially Fc–tm-nT�+–C60��—involved

an energy transfer scenario, in which Fc–tm-1*nT–C60

deactivates via Fc–tm-nT-1*C60. Nevertheless, the positive charge

shifted from nT�+ to Fc producing Fc�+-tm-nT–C60��. The latter

lasted up to 330 ns when replacing 4T with 12T. The longer

lifetimes of Fc�+-tm-nT–C60�� compared to those of

(Fc–nT)�+–C60�� are rationalized by the presence of the tri-

methylene chain, which, in turn, disrupts the conjugation between

the Fc and the nT moieties (Fig. 10). b was evaluated for

Fc–tm-nT–C60 utilizing the corresponding donor acceptor

distances obtained from the optimized structures. In benzonitrile,

for example, a value of 0.10 A�1 was obtained. Notably, the

attenuation factor b is significantly higher than for the corres-

ponding H2P–nT–C60 conjugates, with a value of 0.03 A�1. In the

latter, H2P and nT are directly connected, pointing, once again, to

the critical role of the linkage between the donor and the bridge.

C60–bridge–Fc arrays (27a–c) in which a fulleropyrrolidine

and a ferrocene units are connected via an oligo-p-phenylene-

vinylene bridge, have also been studied (Fig. 11). Photophysical

studies show a complete quenching of the fluorescence

of organic conjugated moieties in 27a–c via energy transfer

to the Fc unit. In the more complex C60/Fc arrays,

the quenching of the C60 moieties is ultrafast in CH2Cl2solution and most likely attributable to electron transfer via

the p-phenylenevinylene wire. In toluene, the dynamic process

of singlet and triplet fullerene quenching can be traced via

time-resolved fluorescence and transient absorption spectro-

scopy and the values of the rate constants are smaller with

increasing donor–acceptor distance. However, a definitive

assignment of the intercomponent quenching mechanism

between the fullerene and the ferrocene moiety (energy or

electron transfer) was not obtained due, probably, to the

competition of the C60–Fc singlet–triplet and triplet–triplet

energy transfer with the charge separation process.47

3. Fullerenes as molecular wires

Turning to recent developments of ideal molecular wires,

several milestones toward molecular-scale electronic devices

have already been passed regarding the transduction of

electrons through single molecules.48–50 Implicit are experi-

mental conditions that guarantee molecular conduction

through a single molecule rather than through an ensemble

of molecules. Moreover, to eliminate cooperativity in trans-

port and to avoid equalizing the conduction through mole-

cules in different conformations are important incentives.

The feasibility of utilizing single molecules as active

elements in electronic devices offers myriad opportunities.

To demonstrate these principles, a series of recent experiments

have documented electron transport through single molecules.

One of the main challenges in this field is the development of

techniques that assist in reproducibly ‘‘wiring up’’ single

molecules. Up to now, scanning probe microscopy techniques—

STM or AFM—have been widely employed in conductance

measurements in single molecule set-ups.51,52 As a viable

alternative mechanical break junctions have emerged in a

series of groundbreaking experiments.53,54 More recently,

other approaches were brought forward. One of these includes

the fabrication of electrodes that are spaced by nanometer

scale gaps. These gaps are sufficiently small to ‘‘wire up’’

molecules in a planar geometry. With the help of these

techniques, Diederich, Calame and co-workers55 have

measured electronic transport through thiolated C60 deriva-

tives. To this end, a two-probe configuration was provided by

the tips of a break junction (Fig. 12). Single functional groups

at the termini of the electrodes serve as anchors and affirm the

trapping of single molecules between the gold contacts. Of

interest is the fact that one may control the coupling of C60 to

the gold electrode by mechanically adjusting the inter-

electrode spacing d in a liquid environment. Obviously, the

environment combined with a certain geometry, exerts a

strong impact on the tunneling rates between the gold elec-

trode and C60. These pioneering results—together with the

capability of modifying the linker groups—afford valuable

insight into the electronic coupling between molecules and

electrodes in molecular devices.

On the other hand, Nishino, Ito and Umezawa56 have

constructed molecular tips for STM which are based on C60.

Those tips were prepared by chemically modifying a corres-

ponding metal tip. As a result, electron tunneling events

involve the outermost single adsorbate probes and the sample

molecule. Importantly, a clear trend was found between the

tunneling current and the tip–sample interactions. Metal

coordination or hydrogen-bonding interactions, for example,

provide the required overlap between the electronic wave

functions of the porphyrin and C60 and, thus, gate/facilitate

the tunneling current. In this context, Ito and co-workers have

been able to detect electron tunneling within single ZnP/C60,

CoP/C60 and H2P/C60 associates (Fig. 13). For instance,

charge-transfer interactions between CoP- and C60-modified

tips involve the highest occupied molecular orbital (HOMO)

of CoP and lowest unoccupied molecular orbital (LUMO) of

the C60 tip. The former and the latter are high and low in

energy, respectively. Such an energy relationship is considered

Fig. 11 Structure of Fc–p-phenylenevinylene–C60 (27).

Fig. 12 Schematic representation of a break junction with a thiolated

C60 molecule anchored to the left electrode. The distance d between the

molecule and the right electrode can be adjusted by opening and

closing the junction.

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to be the basis for directive electron flow when applying a finite

voltage. Particularly striking in these experiments was that

electron tunneling indeed occured exclusively from CoP to

C60—but not in the opposite direction. It has been further

demonstrated that such localized tunneling provides a method

to visualize the frontier orbitals of CoP and evaluate their

energies. Moreover, rectified tunneling was observed in the

polarity dependence of the STM images and the I–V curves.

This, in turn, indicates that the MP/C60 pair constitutes a

molecular rectifier.

A field-effect transistor-like behaviour has recently been

reported in fullerene-substituted mixed-valence bis(ferrocenyl-

ethynyl)ethene derivatives in which the electronic interactions

between the electroactive species are modulated by through-

space intramolecular interactions of the C60 with the

conjugated system.57

Conclusions

One of the major themes in electronics is the construction,

measurement and understanding of the current–voltage

response of an electronic circuit, in which molecular conju-

gates act as conducting elements. In this light, the selected

examples described in this review illustrate the exponentially

increasing interest—experimental, theoretical, and techno-

logical aspects—and potential of molecular wires as multi-

functional building blocks in well-ordered multicomponent

conjugates/hybrids. If a molecular computer is ever to be

built, then it will need molecular wires, and these will have

to be connected to its various components. Molecular wires

possess exciting mechanical, optical and electrical properties

that would seem to make them ideal nanoscale materials.

But despite this great promise, chemists/materials scientists/

physicists have encountered great problems in actually

working with molecular wires. Thus, the systematic explora-

tion of these systems should surely be regarded as break-

throughs in light of implementing molecular wires into

new optoelectronic devices—including molecular electronics,

printable electronics, etc.

Noteworthy is the recent introduction of elegant and versa-

tile protocols concerning the chemical functionalization of

p-conjugated oligomers of precise length and constitution. In

fact, some of the aforementioned limitations can be easily

overcome through the controlled covalent functionalization of

the molecular wire’s termini. This includes, for example,

attaching covalently linked molecular ‘handles’ that serve

ultimately as sources and drains for charge carriers. Previous

work has eluded how the connection between molecular

building blocks and electrodes greatly affects the current–

voltage characteristics. Again, we believe that the control over

handling the linkage emerged as a key asset.

Besides the outreach into the field of molecular electronics,

the design and development of light harvesting, photoconver-

sion, and—as a long term aim—catalytic modules should not

be overseen. They should be capable of self-ordering and self-

assembling into integrated functional units, which will render

it possible to realize efficient artificial photosynthetic systems.

In summary, it is fair to emphasize that the systematic

investigation of molecular wire behavior has, already at a

relatively early stage, played a significant role in the develop-

ment of useful molecular building blocks. If the more techno-

logical problems can be solved, there is an almost unlimited

field of application to be foreseen and eventually molecular

wires may become important building blocks in emerging

technologies. In a more visionary view these nanoscale mole-

cular wires may help minimize computer circuit dimensions

and enhance performance.

Nevertheless, we wish to close with a recent statement by

Nitzan and Ratner48 ‘‘. . .Despite several experimental and

theoretical advances, including the understanding of simple

systems, there is still limited correspondence between experi-

mental and theoretical studies of these systems. . .’’

Acknowledgements

Financial support from the SFB 583, DFG (GU 517/4-1), the

Office of Basic Energy Sciences of the U.S., the MICINN of

Spain (projects CTQ2005-02609/BQU and Consolider-Ingenio

2010 CSD2007-0010, Nanociencia Molecular), the CM

(project P-PPQ-000225-0505) and the EU (FUNMOL

FP7-212942-1) is greatly appreciated.

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